In practice, color images are obtained by exposing silver halides layers, and then developing these layers using developing agents of the paraphenylenediamine type. When these agents are oxidized, in proportion to the development of the silver halides, they react with couplers to form dyes of the azomethane type. Each layer of emulsion in a color photographic material is associated with a coupler that by reaction with an oxidized paraphenylenediamine will produce a primary subtractive color, yellow, magenta or cyan, complementary to the area of sensitivity of the emulsion layer (blue, green or red, respectively). The couplers are added to the emulsion layers or to adjacent layers as solutions in heavy or oily solvents with high boiling point, dispersed in hydrophilic colloids such as gelatin or gelatin derivatives, for example phthalylated gelatin.
Additives other than couplers, for example dyes, or substituted hydroquinones such as dioctyl hydroquinone, can be added to the photographic layers by the same method. These conventional methods for the inclusion of additives are disclosed in Research Disclosure, publication 36544, September 1994, page 527.
The incorporation of some additives, such as couplers, requires auxiliary solvents that are useful during the preparation of the dispersion, but which have subsequently to be removed by any of various known means. This invention relates to dispersions in which the auxiliary solvents are removed by washing.
In general, the introduction of additives involves:
preparation of a fluid dispersion in the high boiling point solvent, the hydrophilic colloid and an auxiliary solvent; PA1 congealing and subdivision of the chilled dispersion to obtain a high contact surface area, and the washing of this dispersion to remove the auxiliary solvent; PA1 storage of the washed subdivided congealed dispersion; and PA1 remelting of the dispersion for use in the preparation of a photographic layer. PA1 (1) forming a fluid dispersion that comprises a hydrophilic colloid, a high-boiling solvent, an auxiliary solvent and a photographic reagent; PA1 (2) chilling, dividing and washing the dispersion to remove the auxiliary solvent from the dispersion. PA1 (3) Additing an aqueous solution of at least one biocide to the washed chilled dispersion.
Although the dispersion is stored at low temperature (below 10.degree. C.), the presence of a hydrophilic colloid of the gelatin type favors the proliferation of micro-organisms. Such microbial growth, which is especially prevalent in summer, can spoil whole batches of dispersion. To prevent such spoilage, biocides and anti-bacterial agents have been employed. However, the use of such biocides meets the following difficulty: the biocide can be added at the time of the preparation of the dispersion, when this is fluid, but it is liable to be removed at the washing stage, leaving the dispersion unprotected during storage. If the biocide is added after washing, the dispersion has to be remelted specially for that purpose. Addition of a further step to an already complex process is not desirable.